I dont think so. It's theoretically hard, cause you can use redundant channels (or not use), take signals from ET200M or not take, make simple or extended diagnostics, use scaling functions standart or self-written - etc.

So memory is a matter of how you write your programm, of course it depends on number of IOs, but indirectly.

Hi NBRM,

It’s very difficult to calculate your required Memory as per your I/O count.

The memory of the S7 400 CPUs can be divided into the following areas:

1. User memory:

A) Load memory:

i) Load Memory intergraded: RAM with battery backup.

ii) Load Memory external: RAM with battery backup or non volatile flash memory.

B) Work Memory:

i) Work Memory Code: RAM with battery backup. For process image I/O (both central and distributed) and diagnostics buffer.

ii) Work memory Data: RAM with battery backup. For local data stack.

2. System memory: RAM with battery backup. For contains Bit memories, Timers counters (managed by operating system), Block stack and interrupt stack.

N.B: You can only expand your load memory up to the CPU specified range.

Load memory contains: (Fig1)

i) The user program

ii) The system data.

The work memory contains: (Fig 2, 3, and 4)

i) Logic blocks

ii) Communication job

iii) Process Image input

iv) Process Image Output

v) Diagnostics buffer.

Required working memory in code/data memory

Size of the process image (inputs): 12 bytes per byte in the process input image in Code memory

Size of the process image (outputs): 12 bytes per byte in the process output image in Code memory

Communication resources (communication jobs): 72 bytes per communication job in Code memory

Size of the diagnostic buffer: 32 bytes per entry in the diagnostic buffer Code memory

Quantity of local data: 1 byte per byte of local data in Data memory

That means:

Your Load memory depends upon your programming part and H/W configuration which is no longer correlated with your I/O count.

Your work memory depends upon your load memory (Logic blocks) + process image I/Os + Diagnostics buffer and communication jobs.

In addition you can follow the link given by Pegaia as “Tomb Rule for estimation of memory occupation in SIMATIC PLC.”